Strong framing protocol for HDLC and other run-length codes

ABSTRACT

Ideal for use in high-speed wide-area networks, such as HDLC as defined in ISO Standard 3309, a strong framing method for data packets significantly improves undetected error probability. In a run-length code type data communication scheme, wherein frame delimiter strings, abort strings and idle strings, are transmitted as control symbols, the method includes providing the frame delimiter string as a multi-symbol construct for delimiting the transmitted data, providing the abort strings and idle strings to respectively include minimum run lengths which are substantially longer than the individual symbols in the multi-symbol construct, transmitting data with control symbols as a bit sequence, and receiving and analyzing the transmitted data at the data-receiving device to detect the presence of bit errors.

FIELD OF THE INVENTION

The present invention relates generally to bit oriented data linkprotocols used in the transmission of information in high-speedwide-area networks and, more particularly, to data transmissionreliability and enhanced framing techniques for High Level Data Control("HDLC" as defined, for example, in ISO Standard 3309).

BACKGROUND OF THE INVENTION

Data communications systems comprise data communications equipment anddata processing equipment connected together by data transmission links.Information is transmitted over these data transmission links in aserial fashion from one device to another in the form of a stream ofinformation which has an identifiable beginning and end. This serialstream of information contains not only data information which includesthe origin and/or destination of the data and information indicating thetype of data being conveyed, but also control information which includesinformation designating the beginning and end of the stream. Beforetransmission, the data information and the control information aremerged together to create a frame, which consists of a data fieldsurrounded by specially coded control symbols.

Bit-oriented data link protocols are a set of predetermined agreementsgoverning the exchange of information. One type of bit-oriented protocolis based on a run-length code which is used to efficiently merge, andsubsequently separate, the control information from the data informationbefore and after transmission on the communications link. The run-lengthof ones is the number of successive ones in a bit stream without anyintervening zero. Similarly, the run-length of zeroes is the number ofsuccessive zeroes in the bit stream without any intervening one. In arun-length coding scheme, special symbols with large run-length are usedas control symbols. A bit stuffing technique is used to ensure that suchsymbols do not appear in the middle of the data field of the frame.Thus, the run length of ones and zeroes provides a way to distinguishbetween the control information and the data information received.

A symmetrical run-length code specifies the maximum run length of bothones and zeroes which can occur during the active state of informationtransmission. Since a symmetrical run-length code guarantees a minimumtransition density of ones and zeroes for all data and control streamsin the active state, clocking information can be encoded. Note that thelimit applies only during the active state. During idle state, there isno upper limit to the run length of ones or zeroes transmitted.

HDLC is also a run-length code. However, it falls into a second class ofrun-length codes referred to as asymmetrical run-length codes.

Asymmetrical run-length codes are those that specify either a maximumrun length of ones or a maximum run length of zeroes in the active,non-idle state. Since asymmetrical run-length codes only guarantee amaximum number of ones in a zero field, or vice versa, they cannoteffectively encode clocking information. HDLC limits the number of onesin a zero field. Asymmetrical run-length codes are DC unbalanced.

In HDLC, a control symbol is placed at the beginning and the end of thedata stream to indicate to the receiver when a new stream of data beginsand to indicate when the stream of data ends. The data stream bounded bythe control symbol is called a packet or a frame. The control symbolused to indicate the beginning of the frame is called "startingdelimiter." Similarly, an "ending delimiter" is used to indicate the endof the frame. Other control symbols are used to abort frames and changeline state. In HDLC, a single control symbol can serve as an endingdelimiter for one frame and as a starting delimiter for another frame,and so it is more appropriately called a "frame delimiter." The framedelimiter is defined as a unique pattern of ones and zeroes comprising01111110--a sequence of six ones preceded and followed by zeroes. Therun length of ones in the frame delimiter is six. The receiverrecognizes this pattern as control information and operates accordingly.The frame, however, also contains data information in the form of onesand zeroes whose pattern can be entirely arbitrary. Consequently, it ispossible that the starting delimiter pattern, 01111110, could appear atany time during the transmission of data information. If the receiverrecognizes this pattern as a frame field instead of the data itrepresents, the data transmission would be destroyed. To prevent thisproblem, an operation called "bit stuffing" is used.

"Bit stuffing" involves altering the original data stream by placing("stuffing") a zero bit in the sequence of ones whenever the run lengthof ones exceeds five. These extra (stuffed) zero bits are removed at thereceiver by observing the run length of ones. Whenever a zero follows asequence of 5 ones, the zero is assumed to be a stuffed bit and isremoved. A larger run length of ones is interpreted as a control symbol.Thus, bit stuffing allows the receiver to distinguish data informationfrom control information. The maximum run length could have been set atany other boundary value. This boundary value has been set to five inHDLC.

While useful for delineating the control stream from the data stream,bit stuffing does not account for data errors which are capable ofdestroying or prematurely terminating a transmission. Consider, forinstance, a data sequence including two zeroes, followed by five onesand another zero (i.e., 00111110). If this sequence experiences an errorduring the transmission which converts the second zero to a one (i.e.,01111110), the device receiving this transmission will recognize it as aframe delimiter control symbol, indicating that the data sequence(frame) has ended or a new data sequence has begun.

Traditional implementations use a checksum or cyclic redundancy check(CRC) to detect such errors. If the checksum that is calculated afterreception of the transmitted data does not correspond to thepre-calculated checksum sent with the data, the receiving deviceattempts to identify and correct the erroneous bit or requests are-transmission. Unfortunately, the effectiveness of suchdetection/correction algorithms depends on to the complexity andbit-length of the checksum sequence. HDLC uses a 16-bit CRC. Theprobability that a falsely created frame delimiter will not be detectedby a HDLC CRC is 2⁻¹⁶ P. Here, P is the probability of a bit error.Since the number of bit errors that result in undetected errors is justone, conventional HDLC framing has a Hamming distance of one. Both theprobability of undetected errors and the Hamming distance of HDLC areunacceptable for many high-speed applications where billions of framesare transmitted every day. Thus, there is a need to either strengthenthe CRC or use a stronger frame delimiter.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art byusing a multi-symbol construct as the frame delimiter. The framingtechnique of the invention is so strong that all noise events thatresult in three or less bit errors can be detected. To create anundetected error, a minimum of four bits must be in error. In otherwords, the Hamming distance is four. This means that the probability ofundetected errors is now reduced to 2⁻¹⁶ P⁴. The bit error probability Pis usually 10⁻⁵ to 10⁻¹⁰. Thus, the present invention realizes animprovement on the order of 10¹⁵ to 10⁻³⁰ over the conventional framingscheme.

Another innovative aspect of the inventive framing technique is that thetransmitters and receivers using the new framing can coexist and workwith those using the conventional HDLC framing technique. The framesusing the inventive strong framing scheme are correctly decoded byexisting HDLC receivers. This property allows a network consisting ofmany transmitters and receivers to be upgraded incrementally and it isnot necessary to replace all of them at once.

According to a preferred embodiment of the present invention, a strongframing method is used for data packets which are transmitted as bitsequences from a data-transmitting device to a data-receiving device ina run-length code type data communication scheme. The method includesproviding the frame delimiter string as a multi-symbol construct fordelimiting the transmitted data, wherein the multi-symbol constructincludes a plurality of individual control symbols each including atleast eight-bits; providing the abort strings and idle strings torespectively include minimum run lengths which are substantially longerthan the individual control symbols in the multi-symbol construct;transmitting informational data with the control symbols as a bitsequence from the data-transmitting device; receiving the transmitteddata at the data-receiving device; and analyzing the received data todetect the presence of bit errors.

The above-described embodiment is merely an exemplary embodiment and isnot intended to represent each of the embodiments of the presentinvention discussed in the specification that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the present invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

FIG. 1 is a block diagram of a system for implementing the presentinvention;

FIG. 2 is a flow chart showing how a transmitter, as shown in FIG. 1,may be implemented in accordance with the present invention;

FIG. 3 is a diagram illustrating how shift registers, used in connectionwith the transmitter referred to with FIG. 2, may be implementedaccording to the present invention;

FIG. 4 is a flow chart showing how a receiver, as shown in FIG. 1, maybe implemented in accordance with the present invention; and

FIG. 5 is a diagram illustrating how the receiver referred to in FIG. 4,may be implemented according to the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will be described in detail herein. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed. On the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, a preferred embodiment of the present inventionis illustrated in block diagram form. FIG. 1 illustrates a two-way datacommunication system having two virtually identical sets of hardware,the master set which is designated as "A" and one of several slave setswhich is designated as "B". The respective communication paths aredesignated as 10 and 12.

Under the control of a master computer 14, data at the master side issent over the communication path 12 through a communication memorycircuit 14 and a transmitter circuit 18. Under the control of a slavecomputer 24, data is received over the communication path 12 at areceiver circuit 20 and, via a communication memory circuit 22, is fedto the slave computer 24. Each computer 14, 24 includes conventionalworking memory 30, 32 for storage of operational code and short-term andlong-term data storage.

Except for the software and hardware changes discussed below inconnection with FIGS. 2-5, the hardware and software architecture isconventional, as may be found in existing HDLC data communicationnetworks. See, for example, the HDLC systems illustrated and describedin U.S. Ser. No. 07/845,673, by Richard L. Szmauz and Anthony Lauck(assigned to the instant assignee) and U.S. Pat. No. 5,007,045, byTsuzuki (assigned to NEC Corporation), both of which are incorporatedherein by reference. Before discussing the exemplary software andhardware particulars which may be used to implement the presentinvention, a general discussion of a preferred strong frame codingapproach is provided below.

In accordance with the present invention, data sent over eithercommunication path 10, 12 uses a frame delimiter which is implemented asa multi-symbol construct. For a desired Hamming Distance ("HD") of four,this delimiter is preferably implemented as a 34-bit frame delimiterinstead of the single-symbol 8-bit delimiter used in conventional HDLCsystems. Thus, rather than using a zero, six ones, followed by a zero(--01111110--), the beginning and end of a frame is preferablydesignated as:

    --01111110 011111110 011111110 01111110--.

Using "F" to represent --01111110--and "A₇ " to represent --011111110--,this multi-symbol frame delimiter may be represented as --F A₇ A₇ F--orby a single letter S. Further, by properly defining the idle stringsequence to a sequence with a run length greater than the length of theframe delimiter S and by defining the abort string sequences to besubstantially longer than one of the individual symbols of themulti-symbol construct, the probability of an undetected error occurringin connection with the frame delimiter is improved over conventionalHDLC networks, which use sixteen-bit checksums, from (P×1/65,536) to (P⁴×1/65,536), where P is the probability of any error occurring in thecommunication link. Thus, the improvement provided with the presentinvention is on the order of P³.

For example, consider an implementation in which only the control stringdefined as a zero, followed by twelve ones, followed by a zero(--01111111111110--), is allowed to be the abort string and is used toprematurely terminate a data transmission, and in which the idle stringsequence is restricted to a minimum run length of 35 and is used toindicate that the link is no longer active (no data is being sent).Using this criteria, a Hamming Distance of four is realized for eachcontrol symbol (abort, idle, frame delimiter). At least four errors arerequired to change one symbol into another symbol as illustrated below:

    ______________________________________                                        SymbolBit-Error                                                               TransitionExamples                                                            ______________________________________                                         ##STR1##                                                                      ##STR2##                                                                      ##STR3##                                                                      ##STR4##                                                                      ##STR5##                                                                      ##STR6##                                                                     ______________________________________                                    

In this illustrative table, "S" represents the above-discussedmulti-symbol frame delimiter --F A₇ A₇ F--, D represents informationaldata, "A" represents the above-discussed abort control symbol, and "I"represents the idle symbol. There is an "E" below each changed bit inthe far fight column to show where the errors occurred to cause thesymbol transition.

In conventional HDLC framing, a single zero may be shared among multiplecontrol symbols. Thus, --011111101111110-- would be considered a validsequence and would be interpreted as a sequence of two F symbols. In theproposed invention, zero sharing is not allowed. The transmitters alwayssend each control symbol with its own zeroes preceding and following thesequence of ones. Thus, to send two F symbols, the transmitter followingthe proposed invention will send 0111111001111110. The receivers willinterpret --011111101111110--as an invalid sequence and indicate anerror condition.

Accordingly, the above implementation may be used to significantlyimprove the data integrity of data transmission, with errors beingdetected as shared zeroes between any of the above-referenced controlsymbols.

Another important aspect of the above-exemplified multi-symbol framedelimiter is the practicability of retrofitting existing systems withthe present invention. Because the frame delimiter F used forconventional HDLC systems is incorporated within the above-exemplifiedframe delimiter S of the present invention, systems having only thetransmitter circuit (and not the receiver circuit) modified toaccommodate this frame delimiter are compatible with conventional HDLCoperation. For example, an unmodified receiver receiving the aboveexemplified multi-symbol construct S simply detects a frame delimiter Ftwice within the decoding process of the multi-symbol construct. As willbe discussed in connection with FIG. 4, the receiver circuit can bemodified so as to be compatible with a conventional HDLC-typetransmitter circuit.

Referring now to FIG. 2, a flow chart shows how the transmitter circuitsof FIG. 1 may be implemented in accordance with the above-describedembodiment of the present invention. Each of the transmitter circuits inFIG. 1 includes data control circuitry, which may be implemented usingconventional digital circuitry with or without a microcomputer as shownin FIG. 3.

Once the flow chart of FIG. 2 is entered (block 40), substantiveexecution begins by immediately sending the strong frame delimiter --FA₇ A₇ F--, depicted as "S" at block 42. At block 44 the transmittercircuit determines whether or not there is data to follow the framedelimiter. As long as there is no data to send, flow returns to block 42and the transmitter circuit continuously sends the frame delimiter S.Alternatively, the transmitter circuit could send an idle stringsequence during this period of inactivity.

From block 44, if the transmitter circuit determines that there is datato follow the frame delimiter, flow proceeds to block 46 where a bitcounter is initialized to zero. This bit counter is used to ensure thatthe five-bit run-length maximum is not exceeded. At block 48, the firstdata bit is shifted from the communication memory in preparation fortransmission over the communication path, and the bit counter isincremented.

At block 50, the transmitter circuit checks if five bits have beenshifted in from the communication memory. If five bits have not beenshifted in from communication memory, then flow proceeds to block 51,where the transmitter circuit determines if there is any more dataintended to be sent out over the data communication path. If there isadditional data to be sent out, flow returns to block 50, where the nextdata bit is shifted from the communication memory, and the bit counteris again incremented. This loop continues until the first five bits areshifted into the transmitter circuit, at which time the path from block50 leads to block 52.

At block 52, a test is performed to determine if the five bits shiftedinto the transmitter circuit are all ones (--11111--). If this is notthe case, flow proceeds to block 53, where the first of the five bits isshifted out to the communication path and the next bit is shifted intothe transmitter circuit. From block 53 flow returns to block 52 todetermine, with this next bit shifted into the transmitter circuit,whether or not this next set of five bits comprises all ones. From block52, flow proceeds to block 54 when each of the five collected bits is aone.

At block 54, each of the five (leading) bits is shifted out to thecommunication path, and the transmitter circuit stuffs a zero bit at theend of this transmitted data. This limits the run length of ones in thedata stream and ensures that the data is not perceived as a controlsymbol, as previously discussed in connection with conventional HDLCsystems. From block 54, flow returns to block 44, for processing anyfurther data to be transmitted onto the communication path.

Referring back to blocks 50 and 51, flow proceeds from block 50 to block51 and then to block 60 when fewer than five bits have been shifted intothe transmitter circuit and there is no more data to send out onto thecommunication path. Thus, at block 60 the remaining bits (the set lessthan five) are shifted out, followed by a multi-symbol frame delimiter(block 62) to indicate "end-of-data." From block 62, flow returns toblock 42 where another multi-symbol frame delimiter is sent to delimitthe next block of data.

FIG. 3 illustrates an exemplary diagrammatic embodiment of thetransmitter circuit of FIG. 1, which one having ordinary skill in theart will recognize can be implemented using conventional digitalcircuitry, for use in connection with the flow chart of FIG. 2. Thecircuit of FIG. 3 includes a framing/bit-stuffing circuit 70 for feedingdata from either a data-shifted memory 72, a frame-delimiter memory 74or a zero-bit memory 76. These memories 72, 74 and 76 are used to senddata out onto the data communication path (10 or 12 of FIG. 1), asdescribed in connection with FIG. 2. The data-shifted memory 72 is usedas described for block 53, the second part of block 54, and block 60 ofFIG. 2. The frame-delimiter memory 74 is used as described for blocks 42and 62, and the zero-bit memory 76 is used as described for the firstpart of block 54. A controller, e.g., a microcomputer, responds to datareceived ("data in") from the communication memory by sending theappropriate control signals, via control bus 80, to theframing/bit-stuffing circuit 70 and the memories 72, 74 and 76. Oneskilled in the art will recognize that the circuit of FIG. 3 is readilyimplemented using conventional digital logic, and/or a microcomputerhaving internal or external memory.

FIG. 4 is the counterpart flow chart for the transmitter circuit flowchart of FIG. 2. Thus, the flow chart of FIG. 4 illustrates how thereceiver circuit of FIG. 1 may be implemented in accordance with thepresent invention. Once the flow chart is entered (block 90),substantive execution begins by immediately shifting n bits from thedata communication path into the receiver bit-register, as depicted atblock 92. The variable n is set equal to the length of the framedelimiter. With strong framing, n is set to 34. However, incompatibility mode, which is explained later, n is set to 8. At block94, the receiver circuit determines whether or not the first n bits ofthe bit-register match the first n bits of the above-describedmulti-symbol construct, abbreviated as "S" in FIG. 4. If the bits in thebit-register does not match those in the multi-symbol construct, thenflow proceeds to block 96, where the leading bit within the bit-registeris discarded and another bit from the communication path is shifted intothe tail end of the bit-register. From block 96, flow returns to block94, where the comparison is repeated for this next set of n bitsreceived from the communication path. This process of shifting one bitat a time into the bit-register continues until a match to the n bits ofthe multi-symbol construct is made.

Once the receiver circuit determines that it has a match, flow proceedsfrom block 94 to block 98 where the receiver circuit enters the datamode. Entering the data mode means that the receiver circuit hasidentified, via block 94, that a new frame is being received. Thus, thereceiver circuit discards the present contents of the bit-register andshifts in the next n bits of data from the communication path.

From block 98, flow proceeds to block 100 to determine if the bits justshifted into the bit-register match the first n bits of the multi-symbolconstruct "S," which is the frame delimiter indicating that the framehas ended. If the bits just shifted into the bit-register represent "S,"flow proceeds to block 102 because the receiver circuit has determinedthat the data mode has ended successfully. Thus, at block 102 thereceiver circuit delivers the previously received data frame for furtherprocessing. From block 102, flow returns to block 98 to determine if anew data frame is being defined.

At block 100, if the bits just shifted into the bit-register do notmatch the first n bits of the multi-symbol construct "S," flow proceedsto block 104 where the receiver circuit determines if bit "destuffing"is required. As previously discussed, bit stuffing involves identifyinga series of five contiguous ones and inserting a trailing zero so as notto confuse data with the control symbols, and bit destuffing involvesidentifying the five contiguous ones and removing the inserted trailingzero. For the present invention, because the above-describedmulti-symbol construct incorporates the conventional HDLC framedelimiter, the same stuffing and destuffing steps are performed. Thus,from block 104, flow proceeds to block 106 if the receiver circuitdetermines that the received data does not require bit destuffing. Atblock 106, the receiver circuit delivers the leading bit from thebit-register as data and shifts into the bit-register the next bit ofdata from the communication path. From block 106, flow returns to block100 to evaluate this next set of n bits in the bit-register.

From block 104, flow proceeds to block 108 if the receiver circuitdetermines that the received data requires bit destuffing. At block 108,the receiver circuit determines if the sixth bit is truly a stuffed zerobit. If the sixth bit is not a stuffed zero bit, then an uncorrectederror has occurred at this point in the serial bit stream, and flowproceeds to block 110, where the current frame is aborted and the datamode is exited. From block 110, flow proceeds to block 96, where thereceiver circuit once again begins searching for the multi-symbolconstruct in the bit-register.

In an alternative embodiment of the receiver circuit, the receiver isrun in a compatibility mode that accepts data generated by aconventional HDLC transmitter. In the compatibility mode, the operationof the receiver is identical to that described earlier except that thevariable n is set to 8. Since the first 8 bits of the multi-symbolconstruct S are 011111110 which correspond to the conventional framedelimiter symbol F, matching the first n bits is equivalent to lookingfor a conventional frame delimiter. The primary advantage of thisembodiment is that the receiver circuit, in accordance with the presentinvention, can be used to receive conventionally-framed HDLC data, forexample, from transmitters which are not yet upgraded with the conceptsembodied by the present invention. Because the integrity of the receiveddata is undermined by the receiver circuit assuming that the sixth bitis not an error, it is preferred that this alternative embodiment of thereceiver circuit be temporary and that the embodiment illustrated inFIG. 4 be used for long-term implementations.

Note that the transmitter does not require any change in operation sincethe frames delimited by the new strong frame delimiter S are accepted aswell formed by the conventional receivers since the F-A₇ -A₇-F-data-F-A₇ -A₇ -F sequence is interpreted as aflag-abort-abort-flag-data-flag-abort-abort-flag sequence. Uponreceiving the first F, a conventional receiver prepares to receive adata packet but aborts the operation upon receiving A₇ which is theabort symbol for conventional HDLC. Upon receiving the second A₇, itaborts again. Upon receiving the second F, the receiver again preparesto receive the data packet and receives the data. Upon receiving thethird F, it considers the packet as valid and delivers the data to themicrocomputer. Subsequent aborts and flag do not affect the data thathas already been delivered and do not result in any loss of data.

From block 108, flow proceeds to block 112 when the receiver circuitdetermines that the sixth bit is truly a stuffed zero bit. At block 112,the receiver circuit delivers these first five (leading) bits as data,discards (or removes) the appended stuffed zero bit, and shifts into thebit-register the next six bits from the communication path. From block112, flow proceeds to block 100 to evaluate the next set of thirty-fourbits in the bit register.

Turning now to FIG. 5, the receiver circuit is illustrated in blockdiagram form. This circuit includes a combinationbit-register/controller 120 and a deframing/bit-unstuffing circuit 122,which may also be implemented using conventional digital circuitry. Thebit-register/controller 120 includes at least thirty-four bits for thecomparisons discussed in connection with blocks 94 and 100 of FIG. 4.Data is shifted into the bit-register portion of thebit-register/controller 120 from the data communication path, andprocessed by the controller portion of the bit-register/controller 120as described in connection with FIG. 4. The controller portion of thebit-register/controller 120 includes control leads ("CONTROL") foroperating the deframing/bit-unstuffing circuit 122 so that it discardsthe stuffed zero bit and other unwanted bits as described, for example,in connection with blocks 96, 98 and 112 of FIG. 4.

Accordingly, the present invention provides an extremely reliablebit-oriented data link protocol for high-integrity transmission of datain high-speed wide-area networks. The protocol is compatible, in eitherthe transmit or receive direction, with conventional HDLC systems asdefined, for example, in ISO Standard 3309. The concepts embodied in thepresent invention may be used with other run-length protocol schemes aswell.

While the inventive system has been particularly shown and describedwith reference to various embodiments and block diagrams, it will berecognized by those skilled in the art that the diagrams andillustrations are intended to teach those skilled in the art how toimplement the preferred embodiment(s) of the present invention. It willbe understood by those skilled in the art that modifications, changesand embellishments may be made to the present invention described abovewithout departing from the spirit and scope thereof, which is defined bythe claims that follow.

What is claimed is:
 1. A strong framing method for transmitting datapackets which are transmitted as bit sequences from a data-transmittingdevice to a data-receiving device in a run-length code type datacommunication scheme, including frame delimiter strings, abort stringsand idle strings as control symbols which are part of the datacommunication scheme, the method comprising:providing the framedelimiter string as a multi-symbol construct for delimiting saidtransmitted data bit packets, the multi-symbol construct includes aplurality of separately-identifiable individual control symbols each ofthe individual symbols including at least eight-bits; providing theabort strings as having minimum run lengths which are longer than theindividual symbols in the multi-symbol construct; providing the idlestrings as having minimum run lengths which are longer than the lengthof the abort string; transmitting informational data with the controlsymbol data as a bit sequence from the data-transmitting device;receiving the transmitted data at the data-receiving device; andanalyzing the received data to detect the presence of bit errors.
 2. Amethod, according to claim 1, wherein said step of analyzing furtherincludes recognizing shared data bits between adjacent control symbolsas errors.
 3. A method, according to claim 1, wherein the step ofproviding the frame delimiter string includes detecting data in an HDLCframe delimiter string.
 4. A method, according to claim 3, furtherincluding converting the received data to an HDLC format.
 5. A method,according to claim 1, wherein the multi-symbol construct is selected soas to provide a Hamming Distance equal to at least four.
 6. A method,according to claim 1, wherein the idle string is longer than the framedelimiter string.
 7. A method for transmitting data packets as bitsequences from a data-transmitting device to a data-receiving device ina run-length code type data communication scheme, includes framedelimiter strings, abort strings and idle strings which are transmittedas control symbols as part of the data communication scheme, the methodcomprising:providing the frame delimiter string as a multi-symbolconstruct for delimiting the transmitted data, the multi-symbolconstruct including at least four separately-identifiable individualsymbols, each of the symbol having at least eight-bits; providing theabort string which has a minimum run length which is substantiallylonger than the individual symbols in the multi-symbol construct;providing the idle string which has a minimum run length which is longerthan the run length of the frame delimiter string; and transmittinginformational data with control symbols as a bit sequence from thedata-transmitting device.
 8. A method for transmitting data packets,according to claim 7, further including converting data from an HDLCformat.
 9. A method for transmitting data packets, according to claim 7,wherein the multi-symbol construct is selected so as to provide aHamming Distance equal to at least four.
 10. A method for transmittingdata packets, according to claim 7, wherein the frame delimiter stringis about thirty-four bits in length.
 11. A method for transmitting datapackets, according to claim 10, wherein the idle string is aboutthirty-five bits in length.
 12. A method for transmitting data packets,according to claim 10, wherein the abort string is about fourteen bitsin length.
 13. A framing protocol arrangement for transmitting datapackets as bit sequences from a data-transmitting device in a run-lengthcode type data communication scheme, including frame delimiter strings,abort strings and idle strings are transmitted as control symbols, theframing protocol arrangement comprising:means for providing the framedelimiter string as a multi-symbol construct for delimiting thetransmitted data, the multi-symbol construct including a plurality ofseparately-identifiable individual symbols having at least eight-bits;means for providing the abort strings to include minimum run lengthswhich are longer than the run length of individual symbols in themulti-symbol construct; means for providing the idle strings to includeminimum run lengths which are longer than the length of the multi-symbolconstruct; a data-transmitting device which transmits data including thecontrol symbols, as bit sequences; and a data-receiving device whichreceives the transmitted data and detect any shared zero bits betweenadjacent control symbols as errors.
 14. A framing protocol arrangement,according to claim 13, further including means for generating an idlestring to indicate an inactive condition.
 15. A framing protocolarrangement, according to claim 14, wherein the frame delimiter stringis transmitted during periods of data transmission inactivity.
 16. Aframing protocol arrangement, according to claim 13, wherein the framedelimiter string includes --01111110 011111110 011111110 01111110--. 17.A method for transmitting data packets as bit sequences from adata-transmitting device to a data-receiving device in a run-length codetype data communication scheme, including abort strings, idle stringsand frame delimiter strings used to delimit informational datatransmitted in the data packets, the framing protocol arrangementcomprising:a transmitter circuit for providing the frame delimiterstring as a multi-symbol construct for delimiting the transmitted data,the multi-symbol construct including at least fourseparately-identifiable individual symbols having at least eight-bits,wherein the abort string includes a minimum run length which issubstantially longer than the individual symbols in the multi-symbolconstruct, and the idle string is longer than frame delimiter string; adata-receiving circuit; a data-transmitting device which transmits data,including the control symbols as a bit sequence from thedata-transmitting device to the data-receiving device; and wherein thedata-receiving device detects as errors any shared zero bits betweenadjacent control symbols.